This invention relates to a convection heat exchanger system. More particularly, it relates to a means for increasing the effectiveness of a convection heat exchanger system with respect to a conventional convection heat exchanger system.
By definition, the effectiveness of a heat exchanger is the ratio of the heat extracted to the maximum possible heat which could be extracted from a source of heat for a given sink temperature. Thus, the effectiveness of a heat exchanger system is a function of the heat transfer coefficient and the temperature driving force.
In a typical conventional convection heat exchanger system, a plurality of heat transfer surfaces in heat flow communication with a heat transfer medium are disposed in stages which are sequentially contacted by a heat-laden gas stream. The temperature of the gas contacting a stage, and thereby the temperature driving force, decreases at each successive stage in the gas flow path of the system. Although high initial temperature driving forces are attainable with this system, the heat transfer coefficient and therefore the quantity of heat which can be transferred per unit of heat transfer area is relatively low.
The temperature driving force experienced by a stage of a heat exchanger system is the difference in temperature between the heat-laden gas at the heat transfer surface of the stage and the temperature of the heat transfer medium at the stage. In general, the larger the temperature driving force, the more effective is the heat exchange at the heat transfer surface.
By way of illustration, heat recovered from a heat-laden gas stream input to a heat exchanger system may be used to generate steam when the heat transfer medium is water. The heat transfer medium of each stage of the heat exchanger may be fed to a common point, as a header. The temperature driving force would be the difference between the gas temperature at the stage and the temperature of the heat transfer medium at the stage.
By positioning the heat transfer surfaces of a conventional convection heat exchanger in heat flow communication with a bed of particulate matter which is supported by a gas distributor and using a heat-laden gas stream for fluidizing the bed, the heat transfer coefficient of the heat exchanger can be increased. However, due to the temperature uniformity associated with a fluidized bed, each of the stages of the heat exchanger system would be subjected to substantially the same temperature. Thus, the effectiveness of having a large temperature driving force would be lost. The uniform temperature of the fluidized bed will decrease as more heat transfer surface area is added to the system.
Although a larger temperature gradient between the input and output temperatures and thereby a larger temperature driving force can be obtained by the use of a multistage fluidized bed, a relatively high pressure loss associated with a conventional fluidized bed severely limits the number of stages which can be used.